In many applications the electrical isolation of circuits is necessary. For example, to permit flexibility in an application involving several sensors that are interconnected in a single apparatus, the outputs of these sensors should each be electrically isolated from a supply line. Furthermore, circuits that are positioned in surroundings exposed to the risk of explosion and that are designed with the so-called intrinsic safety type of protection must be electrically isolated from other circuits. Usually these circuits are isolated from each other by opto-couplers. Here a disadvantage rests in the fact that, for the transmission of fast alternating signals, the opto-couplers must be operated with a great deal of current in order for the output signal not to be delayed and distorted. In particular, the use of this and other known circuit arrangements is not possible for two-wire sensors with, e.g., a 4-20 mA interface, due to the lack of an appropriate current.
Known from U.S. Pat. No. 5,995,252 is a circuit arrangement with which fast alternating signals can be isolated with only a relatively small loss of power. Starting with an input, an input signal is applied over a capacitor and an ohmic resistor to two opto-couplers, one of which responds to positive and one to negative input signal portions. The outputs of the opto-couplers are merged and applied to a buffer, with an ohmic resistor connected in parallel to the buffer, in order to provide an output signal. A disadvantage rests in the fact that this circuit arrangement is only suitable for alternating current signals.